Acrylic or acrylate copolymers are well known in the art and have been used commercially in pressure sensitive adhesive (PSA) compositions for about 50 years (see “Acrylic Adhesives”, D. Satas, Handbook of Pressure Sensitive Adhesive Technology, 2nd Ed., Van Nostrand Reinhold, New York, 1989, pp. 396-456). It is well established that an acrylic copolymer suitable for use as a PSA must have two main characteristics that distinguish it from an acrylic copolymer that is not suitable for use as a PSA.
The first main characteristic of an acrylic copolymer suitable for use in a PSA is that the copolymer will contain a significant fraction of a monomer with a low glass transition temperature (Tg). This monomer will typically have a Tg less than −20° C. and more preferably in the range of −40° C. to −80° C. It is this low Tg monomer that gives the PSA its soft, tacky properties.
The second main characteristic of an acrylic copolymer suitable for use in a PSA is a means of providing reinforcement to the PSA. This reinforcement prevents the PSA from splitting and oozing during use. There are a number of methods for providing this reinforcement, including for example: addition of high Tg monomers to the copolymer; addition of monomers that cause inter-molecular interactions between individual copolymers; covalent crosslinking of the copolymer; and physical crosslinking of the copolymer via graft or block copolymers.
The first, or tacky, characteristic of an acrylic copolymer was described as early as 1933 in German Patent No. 575,327 (Bauer). The reinforcing feature necessary for a suitable acrylic copolymer PSA, however, was not described until 1959 in U.S. Pat. No. 2,884,126 (Ulrich) later reissued as U.S. Pat. No. RE 24,906. This was the first disclosure of the concept of incorporating high Tg functional monomers into the acrylic copolymer for reinforcement, and these monomers, such as acrylic acid and acrylamide, continue to be used widely today for this purpose.
Another effect, however, of incorporating functional monomers into a copolymer is to change the average chemical properties of the copolymer. The chemical properties can affect many performance characteristics of the PSA, such as the ability of a PSA to wet a surface, the ability of a PSA to dissolve or complex additives, and the relative stability of both the PSA and any additives included with the PSA.
In particular, functional monomers can have significant effects on dissolving or complexing small molecule additives. An example of this effect on a PSA is the interaction of iodine with N-vinyl pyrrolidone described in U.S. Pat. No. 4,323,557 (Rosso et. al.). A disadvantage, however, to the general concept of incorporation of functional monomers for adjusting chemical properties is that the functional monomers can also have a strong effect on the physical properties of the copolymer (e.g., reinforcement). Thus the amount of functional monomer that may be incorporated is typically quite limited, since excessive reinforcement will cause a loss of the soft, tacky properties necessary in a PSA.